Method for preparing reactive layered material in graphite form

ABSTRACT

This invention provides a process for preparing a reactive graphite-like layered material with high chemical reactivity while maintaining stability of a base material. In the preparation process according to this invention, first, the treatment for reducing the number of dangling bonds in the vicinity of the vacancy to form an introducing site is conducted by binding atoms together with each other, which atoms are adjacent to a vacancy in a graphite-like layered material. Then, atoms 3 and 4 to be introduced, i.e., a diatomic molecule made of atoms constituting the graphite-like layered material are introduced into the introducing site formed in advance. Then, new bonds are generated between introduced atoms 3, 4 and the graphite-like layered material.

TECHNICAL FIELD

This invention relates to a process for preparing a reactivegraphite-like layered material.

BACKGROUND ART

A graphite-like layered material such as a graphite and a boron nitridelayers or a nanotube composed of the cylindrical structure thereof islow in chemical reactivity. As for an approach to invest graphite-likelayered material with reactivity, conceived may be a technique ofchemical modification with a reactive functional group thereto or atechnique of intentional introduction of defects therein. Among those,as a method for intentionally introducing defects, such a method hasproposed in which dangling bonds (unbonded hands) are formed by cleavingsome of bonds between carbon atoms constituting a carbon nanotube (JP07-172807 A).

This technique was effective as a method for hole opening in a carbonnanotube, initiation of branching or cutting-up therein. However, In themethod, as large crater structures are formed thereby in the carbonnanotube, there is considerable possibility that the base material maybe corroded or deteriorated from the dangling bonds formed therein,which makes it difficult to use the material for a long period.

DISCLOSURE OF THE INVENTION

In view of the above situation, an objective of this invention is toprovide a process for preparing a reactive graphite-like layeredmaterial with high chemical reactivity while maintaining stability of abase material.

According to an aspect of this invention, there is provided a processfor preparing a reactive graphite-like layered material comprising thesteps of:

-   -   binding atoms having a dangling bond together with each other        which are adjacent to a vacancy included in a graphite-like        layered material, for reducing the number of dangling bonds in        the vicinity of said vacancy to form an introducing site;    -   introducing a molecule or atom constituting the graphite-like        layered material into the introducing site; and    -   generating a new bond between the introduced molecule or atom        and the graphite-like layered material.

In this invention, a reactive graphite-like layered material refers to agraphite-like layered material possessing chemical reactivity. Agraphite-like layered material refers to a mono- or multi-layeredmaterial having a hexagonal main framework, such as graphite and h-BN(hexagonal boron nitride).

In the process for preparing a reactive graphite-like layered materialaccording to this invention, an atom adjacent to a vacancy in agraphite-like layered material used as a base material have a danglingbond being energetically unstable. The preparation process according tothis invention comprises the step of binding the dangling bonds togetherwith each other, and thereby forming an introducing site for a new atomor molecule. The step is a step for affording structural relaxation to abase material.

The term “structural relaxation” as used herein refers to reducing aninternal energy of a system, whereby the number of dangling bonds in thesystem is reduced. When relaxing the vicinity of a vacancy, it canprovide a structure having a metastable binding state although it isless stable than a graphite-like honeycomb structure. The structure isan introducing site.

The process further comprises the steps of introducing an atom ormolecule constituting a graphite-like layered material into theintroducing site thus formed and then generating a new bond between theintroduced molecule or atom and the graphite-like layered material. Inthe preparation process according to this invention, after anintroducing site is formed in the vicinity of a vacancy in a basematerial, a new atom or molecule is introduced thereinto. Therefore,once the introduced atom or molecule is bound to the base material, ametastable structure is formed instead of recovering a graphite-likehoneycomb structure. The metastable structure resulted is structurallyrelaxed to keep away from spontaneous vanishing. The preparation processaccording to this invention can, therefore, stably form a structurehaving high chemical reactivity without any use of chemicalmodifications with an element other than the components of thegraphite-like layered material.

In the preparation process of this invention, the step of forming anintroducing site or the step of generating a new bond therein maycomprise the step of conducting the annealing or photoexciting treatmentto the graphite-like layered material, which treatment allows anintroducing site or new bond to be more effectively formed and to moreeffectively prevent the framework structure of the base material frombeing deteriorated.

According to another aspect of this invention, there is provided aprocess for preparing a reactive graphite-like layered materialcomprising the steps of:

-   -   forming a vacancy in a graphite-like layered material;    -   reducing the number of dangling bonds in the vicinity of said        vacancy by binding atoms adjacent to said vacancy together with        each other to form an introducing site;    -   introducing a molecule or atom constituting said graphite-like        layered material into said introducing site; and    -   generating a new bond between said introduced molecule or atom        and said graphite-like layered material.

The preparation process according to this invention comprises the stepof forming a vacancy in a base material having a graphite-like honeycombstructure, so that a chemically reactive structure can be effectivelyformed. The number of atoms per a vacancy can be controlled to prevent abase material from being deteriorated.

In the preparation process of this invention, said step of forming avacancy may comprise the step of irradiating said graphite-like layeredmaterial with an electron beam. With use of such procedure, a vacancycan be formed further effectively therein.

In the preparation process of this invention, such constitution that anatomic vacancy number per a vacancy is one or two may be employed. Whenan atomic vacancy number is two, a resulted structure with chemicalreactivity is accompanied by no dangling bonds, so that a reaction of adangling bond with an impurity or the like or breakage of achemical-bond network in a base material can be suppressed thereby. It,therefore, allows a reactive graphite-like layered material in whichreduction in mechanical strength of the base material is inhibited to bestably prepared. Even when an atomic vacancy number is one, a structurewith reactivity may be achieved with such structure with no danglingbond.

In the preparation process of this invention, the graphite-like layeredmaterial may include graphite, or alternatively it may comprise mainlynitrogen and boron atoms. Since these materials have a graphite-likehoneycomb structure, they can be effectively activated by thepreparation process according to this invention.

In the process for preparing a reactive graphite-like layered materialof this invention, said graphite-like layered material may constitute aside wall of a nanotube. Thus, in such a case, a nanotube being inferiorin chemical reactivity can be activated therewith, while breakage of achemical-bond network of the base material can be repressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating an embodiment of a process for preparing areactive graphite-like layered material according to this invention,showing an example of the step of forming a vacancy.

FIG. 2 is a view illustrating an embodiment of a process for preparing areactive graphite-like layered material according to this invention,showing an example of a vacancy formed in the layered material.

FIG. 3 is a view illustrating an embodiment of a process for preparing areactive graphite-like layered material according to this invention,showing an example of an introducing site formed in the layeredmaterial.

FIG. 4 is a view illustrating an embodiment of a process for preparing areactive graphite-like layered material according to this invention,showing an example for the step of introduction of a molecule or atomconstituting the graphite-like layered material into an introducingsite.

FIG. 5 is a view illustrating an embodiment of a process for preparing areactive graphite-like layered material according to this invention,showing an example of a structure with chemical reactivity formed in thelayered material.

FIG. 6 is a view explaining another embodiment of a process forpreparing a reactive graphite-like layered material according to thisinvention, showing another example for the step of forming a vacancy.

FIG. 7 is a view explaining another embodiment of a process forpreparing a reactive graphite-like layered material according to thisinvention, showing an example of a structure exhibiting chemicalreactivity formed in the layered material.

The symbols in FIGS. 1, 4 and 7 have the following meanings; 1: adetached atom, 2: a detached atom, 3: an introduced atom, 4: anintroduced atom, and 5: a detached atom.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be explained preferred embodiments of this inventionhereafter.

FIG. 1 illustrates a honeycomb structure of a graphite-like layeredmaterial used as a base material. A material having a hexagonal layeredstructure such as graphite and h-BN (hexagonal boron nitride) may beemployed as the base material therefor. The base material may possessmono- or multi-layered structure, and it may be a compound shaped in asheet form or a compound shaped in cylinder form. For instance, as for alayered compound. HOPG (highly oriented layered graphite) may be used.As for a compound in cylindrical form, for example, a compound having ananotube structure may be used as a base material. Alternatively,hexagonal BC₂N may be also used.

In a graphite-like layered material having a framework structure shownin FIG. 1, a vacancy is formed. During the step, an atomic vacancynumber per a vacancy is to be controlled. As the larger a vacancy sizeis, the more easily the base material is deteriorated. For example, anatomic vacancy number per a vacancy may be within 6. Furthermore, anatomic vacancy number per a vacancy may be preferably limited to 2 orless. Thus, as described below, graphite-like layered material can beeffectively invested with chemical reactivity while preventing thestructure of the base material from being deteriorated. There will bedescribed a process for preparing a reactive graphite-like layeredmaterial according to this preferable embodiment, by way of an examplewhere an atomic vacancy number per a vacancy is one or two.

In the structure shown in FIG. 1, when adjacent two atoms such asdetached atoms 1 and 2 are eliminated, the structure shown in FIG. 2 isobtained therefrom. The structure shown in FIG. 2 has a vacancy whereelimination of two adjacent atoms are detached away. In the case wherethe base material is a graphite material, two adjacent carbon atoms aredetached, while in the case where it is a h-BN material, adjacent onenitrogen and one boron atoms are detached.

Electron beam irradiation may be used as means for forming a vacancy ina graphite-like layered material. Thus, with use of such technique, anatomic vacancy number per a vacancy can be effectively controlled. As amethod of electron beam irradiation, such methods, for example,disclosed in JP 10-139411 A or in Hidefumi Hiura, J. Materials ResearchVol. 16, p. 1287 (2001) can be employed.

A method for irradiation with ions of inert gas having a comparableweight to a weight of two atoms of base-material (e.g., He, Ne or Argases).may be employed as an alternative method for forming a vacancy ina graphite-like layered material used as a base material

Next, the step for structural relaxation is effected for the vacancyformed in the base material. It is because there are numbers of danglingbonds on atoms in the vicinity of the vacancy, which makes the vacancyenergetically unstable. When structural relaxation being conducted,atoms in the vicinity of the vacancy are bound together with each otherto reduce the number of dangling bonds, and also to suppress destructionof the sample such as vacancy expansion.

For instance, such a technique as thermal annealing may be used as amethod for conducting structural relaxation. Annealing may be effectedby, for example, treating at a temperature of 1000 K or higher for 10min to 5 hours. In place of annealing, local photoexcitation may beapplied to induce atomic motion therein. In the case of photoexcitation,it may be conducted by treatment at room temperature, for instance, forduration of from 10⁻¹⁴ sec to 10⁻¹² sec.

Thus, an introducing site shown in FIG. 3 can be obtained. The extent ofreduction in the number of dangling bonds in the base material can beevaluated, for example, based on results of analysis of an infraredspectrum, observation by means of STM (scanning tunneling microscopy) ormeasurement of variation in a vibration frequency of atomic nucleartherein.

Then, a molecule or atom of the base material component is introducedinto the introducing site resulted. In FIG. 4, example is illustrated inwhich a diatomic molecule composed of introduced atoms 3 and 4 isintroduced thereinto. For example, in the case when the base material isa graphite-based material, a C₂ molecule is fed thereinto. If themolecule is fed at the stage that atoms adjacent to the vacancy have notbeen bound together with each other as shown in FIG. 2, the structure isreturned to that shown in FIG. 1. In contrast, a C₂ molecule is fedafter atoms adjacent to the vacancy are bound together with each otherin this embodiment, and thus the energetically stable structure as shownin FIG. 4 will be attained. Typically, introduced atoms 3 and 4 areintroduced such that the bond axis of the C₂ molecule is alignedparallel to the major axis of the introducing site. The introduced C₂molecule may be obtained by decomposing a hydrocarbon molecule such asethylene and acetylene by means of a plasma or the like. For Instance, amethod described in JP. 2001-262343 A may be employed. In the case whenthe base material is a BN-based material, for example, employed may besuch technique that a mixed gas of nitrogen and HfB₂ is excited by aplasma to generate a BN molecule, which is then introduced into theintroducing site of the base material.

As shown in FIG. 4, after introducing the diatomic molecule composed ofintroduced atoms 3 and 4, the step of structural relaxation is againeffectured. As described above, such method as annealing at 1000 K orhigher or photoexcitation may be employed. Thus, a new bond can begenerated between the introduced molecule or atom and the base materialthereby.

The structure thus obtained is just the structure presented in FIG. 5.The structure shown in FIG. 5 comprises two five-membered rings and twoseven-membered rings. The two seven-membered rings comprise a singlebond connecting the apices of the five-membered rings and two singlebonds each linking between an apex and an atom adjacent to the apex, andwhereby the bond axis between introduced atoms 3 and 4 is rearranged tobe in a direction rotated by 90° to the bond axis between detached atoms1 and 2. The structure as shown in FIG. 5 is a metastable Stone-Walestype structure. For example, when in a BN material there exists theStone-Wales type structure presented in FIG. 5, chemical bonds betweenboron atoms and between nitrogen atoms are formed. Thus, theseπ-electron orbitals attributed to the bond between the same elementatoms generate levels in a bandgap of boron nitride, which will endowthe base material with chemical reactivity. Formation of the structureshown in FIG. 5 within the base material may be validated by measurementof the amount of new atoms or molecules introduced into the introducingsite or observation of infrared absorption by means of FT-IR (Fouriertransform infrared spectroscopy), Raman spectrometry or the like.

As an activation energy necessary to convert the Stone-Wales type defectpresented in FIG. 5 into the structure shown in FIG. 1 is several eV,there occurs no spontaneous vanishing thereof, so that it is a stablestructure. Furthermore, as there are no dangling bonds accompanying withthe structure, breakage of a chemical bond in the base material itselfor associated deterioration in the material is hard to be initiated whenthe chemical reaction therein occurs, which leads to a longer life.

A vacancy formed when irradiating the base material having the structureas shown in FIG. 1 with an electron beam may be not only a diatomicvacancy illustrated in FIG. 2, but also a monoatomic vacancy shown inFIG. 6. For example, when using a BN compound as a base material, as theB atom tends to be energetically detached with more ease than another,formed therefrom is not only the structure illustrated in FIG. 2 butalso that shown in FIG. 6. In such a case, the structure illustrated inFIG. 7 is formed by annealing, photo-irradiation or the like. Sincedetached atom 5 has a dangling bond in FIG. 7, electron beam irradiationis again conducted to remove the detached atom 5. After removal ofdetached atom 5, the step for structural relaxation with such asannealing is again effected to give the structure shown in FIG. 3. Thus,in the subsequent process similar to the case having a diatomic vacancy,a Stone-Wales type defect with chemical reactivity (FIG. 5) can beformed.

Thus, in this embodiment, even when vacancies obtained by irradiating agraphite-like layered material as a base material with an electron beamare a mixture of diatomic and monoatomic vacancies, the above steps canbe repeatedly applied to invest them with chemical reactivity.

As aforementioned with referring to FIGS. 1 to 5, a process forpreparing a reactive graphite-like layered material according to thisembodiment comprises the steps of:

-   -   forming a eight-membered ring in a graphite-like layered        material;    -   introducing two atoms or a diatomic molecule into the        eight-membered ring; and    -   forming two seven-membered rings and two five-membered rings        from the eight-membered ring, a pair of five-membered rings each        of which shares one of two parallel single bonds of the        eight-membered ring parallel to its main axis with the        eight-membered ring, and the two atoms or the diatomic molecule.        In addition, when a monoatomic vacancy is formed, the process        further comprises the step of forming a nine-membered ring in a        graphite-like layered material.

Furthermore, the technical field to which a reactive graphite-likelayered material prepared according to this embodiment can be applied isnot limited to any specific targets. For example, utilizing its higherreactivity, it may be effectively used as an adsorbent or carrier forcatalyst.

EXAMPLES Embodiment 1

In embodiment 1, graphite is used as a base material. The base materialhas a honeycomb skeletal structure as shown in FIG. 1.

At first, a graphite having a framework structure shown in FIG. 1 isirradiated with an electron beam. For example, when irradiating the basematerial with a 4 keV electron beam under a reduced pressure at about10⁻⁵ Pa, a current is chosen within range of 10 mA to 40 mA for a samplewith a size of 5 mm×5 mm×2 mm, and an irradiation time is limited to 1min or less. Thus, detached atoms 1 and 2, i.e., two adjacent carbonatoms, are eliminated to form a diatomic vacancy shown in FIG. 2 in thebase material.

Next, the base material in which the vacancy is formed is annealed. Forexample, annealing is conducted under such a condition of at 1000 K for30 min. Thus, carbon atoms in the vicinity of the vacancy are boundtogether with each other thereby to obtain the eight-membered ring shownin FIG. 3, which becomes an introducing site.

Then, into the introducing site shown in FIG. 3 is introduced a diatomicmolecule composed of introduced atoms 3 and 4, i.e., a C₂ molecule, andwhereby the structure of FIG. 4 is produced in the base material. The C₂molecule is generated by plasma decomposition of ethylene or acetylenegas and then contacted with the base material for introduction.

Sequentially, the base material into which the C₂ molecule has beenintroduced is again annealed. For example, annealing is conducted undersuch a condition of at 1000 K for 30 min, and whereby a Stone-Wales typedefect shown in FIG. 5 is produced in the base material.

The reactive graphite thus obtained has the structure shown in FIG. 5 inits skeleton, so that it possesses chemical reactivity while maintainingits mechanical strength.

The fact that a Stone-Wales type defect with the structure shown in FIG.5 which is formed in graphite has chemical reactivity may be confirmedfrom the evidence that chemical bond strength for atoms in the vicinityof the Stone-Wales type defect is higher than those for other atoms, asdescribed in Sara Letardi, Massimo Celino, Fabrizio Cleri, VittorioRosato, Surface Science Vol. 496, p. 33 (2002).

Embodiment 2

In embodiment 2, h-BN is used as a base material. In embodiment 2, thebase material also has the honeycomb skeletal structure shown in FIG. 1.

At first, in similar manner to that of embodiment 1, the base materialis irradiated with an electron beam. Thus, one boron atom and onenitrogen atom neighboring to each other are detached to give a vacancyas shown in FIG. 2. Next, by the similar way to embodiment 1, annealingis carried out to form an introducing site shown in FIG. 3. After that,a gas mixture of nitrogen and HfB₂ is excited by a plasma to generate aBN molecule, which is then contacted with the base material forintroduction into the introducing site. The structure as shown in FIG. 4thus obtained is again annealed to form a Stone-Wales type defect shownin FIG. 5.

Similarly, in embodiment 2, the reactive h-BN obtained possesseschemical reactivity while maintaining its mechanical strength.

INDUSTRIAL APPLICABILITY

As explained above, according to this invention, a process forconsistently preparing a reactive graphite-like layered material withhigh chemical reactivity while maintaining stability of a base materialcan be effectuated by employing a process comprising a series of thesteps of binding atoms having a dangling bond together with each otherwhich are adjacent to a vacancy included in a graphite-like layeredmaterial, for reducing the number of dangling bonds in the vicinity ofthe vacancy to form an introducing site;

-   -   introducing a molecule or atom constituting the graphite-like        layered material into the introducing site; and    -   generating a new bond between the introduced molecule or atom        and the graphite-like layered material.

1. A process for preparing a reactive graphite-like layered material,which is mono- or multi-layered material having a hexagonal mainframework with chemical reactivity, from a graphite-like layeredmaterial referring to a mono- or multi-layered material having ahexagonal main framework comprising the steps of: binding atoms having adangling bond together with each other which are adjacent to a vacancyincluded in said graphite-like layered material, for reducing the numberof dangling bonds in the vicinity of said vacancy to form an introducingsite; introducing a molecule or atom constituting the graphite-likelayered material into the introducing site; and generating a new bondbetween the introduced molecule or atom and the graphite-like layeredmaterial.
 2. A process for preparing a reactive graphite-like layeredmaterial, which is a mono- or multi-layered material having a hexagonalmain framework with chemical reactivity, from a graphite-like layeredmaterial referring to a mono- or multi-layered material having ahexagonal main framework comprising the steps of: forming a vacancy in agraphite-like layered material; reducing the number of dangling bonds inthe vicinity of said vacancy by binding atoms adjacent to said vacancytogether with each other to form an introducing site; introducing amolecule or atom constituting said graphite-like layered material intothe introducing site; and generating a new bond between said introducedmolecule or atom and said graphite-like layered material.
 3. The processas claimed in claim 2, wherein the step of forming said-vacancycomprises the step of irradiating said graphite-like layered materialwith an electron beam.
 4. The process claimed in claim 1, wherein saidstep of forming an introducing site or said step of generating a newbond comprises the step of conducting annealing or photoexcitingtreatment to the graphite-like layered material.
 5. The process claimedin claim 1, wherein said graphite-like layered material includesgraphite.
 6. The process claimed in claim 1, wherein said graphite-likelayered material comprises mainly nitrogen and boron atoms.
 7. Theprocess claimed in claim 1, wherein said graphite-like layered materialconstitutes a side wall of a nanotube.
 8. The process claimed in claim2, wherein said step of forming an introducing site or said step ofgenerating a new bond comprises the step of conducting annealing orphotoexciting treatment to the graphite-like layered material.
 9. Theprocess claimed in claim 2, wherein said graphite-like layered materialincludes graphite.
 10. The process claimed in claim 2, wherein saidgraphite-like layered material comprises mainly nitrogen and boronatoms.
 11. The process claimed in claim 2, wherein said graphite-likelayered material constitutes a side wall of a nanotube.
 12. The processclaimed in claim 1, wherein said vacancy is formed in advance by meansof irradiating said graphite-like layered material with an electronbeam.